1. Field of the Invention
The invention relates to a trench style bump and its fabrication method, and more particular, to an application of employing the trench style bump into flip chip technique, such as chip on glass (COG) and ball grid array (BGA).
2. Description of the Related Art
Integration circuits (ICs) have reached every part of our daily lives. However, the process of fabricating an IC is very complex. Hundreds of steps are needed for making an IC. The fabrication normally takes one or two month to complete. The IC industry is a high technology industry including four main branches: IC design, wafer fabrication, wafer testing, and wafer packaging. Thus, IC industry is not only an advanced technology industry, but also a high risky industry which needs a tremendous capital to maintain.
In a conventional package of an IC chip, a lead frame is used to electrically connect a semiconductor and an external wire of the package. However, as the integrated circuit grows more and more delicate and complex, the required wire increases greatly in the same package volume. The traditional lead frame technique can not meet the requirement at all. Therefore, to develop a new package which contains and connects more conducting wires are required.
One of the package technique which accommodates a large number of wires is BGA package. A BGA package is normally in a square shape in which the conducting terminal is in a form a solder ball. This terminal is designed and installed on a bonding pad of a printed wire board (PWB), a printed circuit board (PCB), or some other devices to connected.
In practice, a conventional BGA is a miniature of a multi-layered PCB, which comprises an IC chip, to electrically connect each other via various ways. The connections between conductors are achieved by via or plug.
Another design which accommodates and connect a large number of wires in a limited region is glass on chip (COG). The wiring or circuit is built in a glass substrate. Through some conducting device or terminal on the chip, for example, gold bump, the chip is coupled directly to the circuit in the glass substrate.
FIG. 1a to FIG. 1f show a conventional method of fabricating a gold bump. Referring to FIG. 1a, an aluminum (Al) pad 102 and a passivation layer 104 are formed on a silicon chip 100. A wafer cleaning step is performed on the silicon chip 100.
In FIG. 1b, an under bump material (UMB) 106 which comprises a titanium tungsten layer (TiW) 106A and a gold layer 106B is formed over the silicon chip 100.
In FIG. 1c, using a photo-mask, a photo-resist layer 108 with an opening 110 aligned with the Al pad 102 therein is formed over the UBM 106. In FIG. 1d, a gold layer 110A is formed to fill the opening 110.
In FIG. 1e, the photo-resist layer 108 is removed, and then the UBM 106 is removed to complete the formation of a gold bump 106B shown as FIG. 1f.
FIG. 2a to FIG. 2b show a conventional method of employing the above metioned gold bump structure a flip chip technique. In FIG. 2a, a substrate 200, for example, a glass substrate or a printed circuit board is provided. A cleaning process is performed on the substrate 200 first. On the substrate 200, an anisotropic conductive film (ACF) or an anisotropic conductive paste (ACA) 202 is formed. In the figure, the particles contained by the ACF/ACA 202 represent the conductive particles.
In FIG. 2b, a silicon chip 100 having a gold bump 110B as described above is laminated on the substrate 200. It is known that the substrate 200 normally includes an organic material. In the case of printed circuit board, bismalemide triazine (BT) compound is used to form a printed circuit board. This kind of material is easily bent and deformed due to thermal expansion, moisture, or inappropriate handle. As shown in the figure, the deformation causes a bad contact between the gold bump and the substrate. Consequently, an open circuit is very likely to happen. In addition, if the surface level of the gold bump 100 is formed non-uniformly, the open circuit is even more obvious.